The experiments in this proposal address the general question of what is the relative importance of a cell's lineage vs. its development environment in determining its characteristics as an adult neuron. Many experiments have shown, in lower vertebrates, that by the time of neural tube closure, the early nervous system is a functional mosaic of cells whose developmental fates are already largely intrinsic to the cells themselves. Evidence assembled during the current grant period suggests that, through the use of genetic mosaics, one can hope to study these same questions in the mouse. Our quantitative analyses of various neuronal populations in adult chimeric mice have lead to the finding that at least two populations, the cerebellar Purkinje cells and the motor neurons of the facial nerve nucleus, are properly thought of as a small number of developmental clones of cells. For example in C57BL/6 mice the entire adult Purkinje cell population is made up to 20 clones of 9,200 Purkinje cells each. To pursue these observations I will look for evidence that cell types known to be related to Purkinje cells (such as the Golgi cells of the cerebellar cortex) share common or desparate lineages. Second, having found several inbred strains of mice with differnt numbers of Purkinje cells, I have determined that for two of them, C3H/HeJ and C57BL/6 both the number of developmental clones and the number of cells per single clone are different. Chimeras will be constructed to analyze F1 hybrid of these two strains, B6C3F1, for clone size and number. The results will give additional insights into the genetic factors that may control these values. Third other neuronal cell populations will be examined for evidence of cell lineage relationships. Our principal efforts in this endeavor will be directed at the spinal motor neurons. Finally, to allow the expansion of this type of analysis into regions of the nervous system which are currently inaccessible (e.g. neural retinal, cerebral cortex, essentially any region with small cells) I will attempt to develop a strain of mouse which carries a gene, introduced by microinjection, whose gene product will be easily detectable in all of the cells of the nervous system. Studies such as these on the early restriction of cell fates and the mechanisms involved in control of cell number in the early CNS are rarely approached in a mammal. The use of the mouse as a model furthers my belief that the findings will bear directly on human development and developmental disorders.